U.S. Pat. Nos. 4,237,625 and 4,358,902 in which one of the patentees is the applicant herein, disclose sole-and-heel structures with one or more cavities in the sole portion and one or more cavities in the heel portion. The cavity or cavities in the sole portion are located immediately behind the foot's metatarsal region. The cavities are covered by a thin material. Pressurized fluid is delivered to these cavities through a suitable valve. The pressurized fluid has a ballooning effect on the thin cavity covers causing them to bulge below the main plane of the sole-and-heel structure. These bulges come into contact with the pavement. At rest, the foot is cushioned comfortably on the pressurized fluid in the bulges. In moving, fluid under pressure, alternates through a passageway between the heel and metatarsal cavities, producing an alternate lifting effect. The bulges facilitate movement by providing forward thrust to the heel and to the metatarsal region.
The sole-and-heel structure must have high wear resistant capabilities so as not to deteriorate rapidly as it contacts the pavement. The bulges in the structures described in these prior art patents are thin in order to balloon out as the result of pressurization. The thinness causes such sole-and-heel structures to have a tendency to wear out more quickly in the areas of the bulges.
Furthermore, the valve itself makes these prior art structures expensive to make. Also, the pressurized fluid tends to escape so that fluid would have to be added from time to time. Finally, the pressurized fluid causes the walls of the cavities in these sole-and-heel structures to be under constant stress, even during nonuse of the shoes incorporating such structure.